Recently, excellent thermal resistance, excellent mechanical properties, and excellent electrical properties are required more than ever in development of organic materials in an electronics field such as a semiconductor, a crystal liquid panel, and a printed wiring board; an environmental field such as a bag filter; a space and aviation field, and the like. For example, in the electronics field, this is because internal devices and batteries therein are reduced in size in accordance with a reduction in size and weight and an increase in wiring density of a mobile phone and a personal computer, resulting in an increased temperature due to internal heat accumulation during use. To solve such a problem, a polyimide resin has been developed and used in various forms such as a membrane, a film, a molded product, a non-woven fabric and a paper in each field. As a new approach, recently, nano-order-sized fibers (nanofibers) of a polyimide with a fiber diameter of 1 μm or less have been examined. As methods for producing an aggregate of fibers with a small fiber diameter, there are a conjugate spinning method, a high-speed spinning method and an electro-spinning method. Among them, the electro-spinning method makes it possible to spin fibers more easily and in a more simple process compared to the other methods. In the electro-spinning method, a liquid (e.g. a solution containing a polymer to form fibers; and a melted polymer) which is charged by applying a high voltage is drawn toward a counter electrode to form fibers. The polymer to form fibers is drawn and forms fibers during drawing toward the counter electrode. The fiber is formed by evaporating a solvent in the case of using a solution containing a polymer which forms fibers, or the fiber is formed by cooling or chemical hardening in the case of using a melted polymer. The obtained fibers is collected on a collecting substrate which is placed according to need, and further, the obtained fibers can be separated therefrom to be used as an aggregate of fibers if required. In addition, since it is possible to directly obtain an aggregate of fibers in the form of a non-woven fabric, there is no need to form an aggregate of fibers after fibers are spun as in the other methods (e.g. refer to Japanese Examined Patent Laid-open Publication No. S48-1466, Japanese Patent Laid-open Publications No. S63-145465 and No. 2002-249966).
As nanofibers using a polyimide resin, it has been proposed a polyamic acid non-woven fabric with an average fiber diameter in the range of 0.001 μm to 1 μm which is obtained by using a thermosetting polyimide comprising a general aromatic tetracarboxylic acid and a general aromatic diamine, and the polyimide non-woven fabric obtained by imidizing the polyamic acid non-woven fabric (Japanese Patent Laid-open Publication No. 2004-308031); and a separator for a lithium secondary battery which is composed of polyimide ultrafine fibers with a fiber diameter of 1 μm or less which is obtained by using a solvent-soluble polyimide (Japanese Patent Laid-open Publication No. 2005-19026). However, they do not sufficiently satisfy thermal dimensional stability such as coefficient of linear expansion required in the fields of use.